Since microwaves have the potential to heat both peripheral and deep tissues, chronic exposure to low energy levels (less than 10 mW/cm square) may have consequences for warm-blooded species to regulate their body temperature. In particular, the normal development of thermoregulatory response systems may be compromised if such exposure occurs in utero or during the first few weeks after birth, when homeothermy is not yet established. The major aim of this study is to determine if a critical period exists in the growth of the immature animal when microwave exposure at selected ambient temperatures can significantly alter physiological and/or behavioral thermoregulatory abilities. Pregnant rats receive daily (8h/day) exposure to continuous wave microwaves (2450 MHz at 1 or 5 mW/cm square) or sham exposure at controlled ambient temperatures (25 or 35 C) from day 2 to 19 of gestation. Neonates are microwave or sham exposed to the same conditions from 6 to 16 days of age. Tests of physiological thermoregulation (oxygen consumption, colonic and skin temperatures) in 5-6 day old neonates indicate that probably no unusual changes in thermoregulatory function are produced by prenatal exposure. The growth rate of both sham- and microwave-exposed neonates is less than normal but physiological thermoregulatory responses approach normal levels in those exposed to microwaves (5 mW/cm square), particularly in the cool (25 C) environment. Proposed experiments will 1) examine the growth rate factor (by supplementary feeding during exposure), 2) confirm that prenatal exposure is an unimportant variable (by testing 2-3 day old neonates), 3) assess the consequences of microwave and sham exposure during days 2-5 of life, 4) determine the equivalence between microwave intensity and ambient temperature for neonates of different ages, and 5) ascertain if there is a critical period between birth and weaning when microwave exposure produces unusual changes in thermoregulatory capability. Evidence for impaired thermoregulatory function attributable to microwave exposure will generate concern that human infants should be protected from exposure to low intensity microwave fields; alternatively, evidence for enhanced thermoregulatory capability may indicate the feasibility of using microwaves for the incubation of immature humans and other mammalian species.